Occupant protection systems for use in vehicles are known in the art. One example type of a protection system includes an actuatable inflatable restraint device, which has an inflatable restraint cushion. The inflatable cushion is commonly referred to as an air bag and the device is referred to as an air bag module.
A controller determines whether the air bag module is to be actuated to inflate the air bag within a vehicle passenger compartment. The air bag module is actuated by the controller upon the occurrence of a predetermined condition for which a vehicle occupant is to be cushioned by the air bag. For example, when a crash sensor that is operatively connected to the controller senses a vehicle condition indicative of a vehicle crash condition, the air bag module is actuated.
Actuation of the air bag module includes initiation of a source of inflation fluid. An inflation fluid source includes gas generating material and/or a gas storage container. In one example type, fluid source initiation involves ignition of an igniter via electrical actuation. Often the igniter is a pyrotechnic squib. When an air bag module containing a squib is to be actuated, an electric current of sufficient magnitude and duration is passed through the squib to ignite the squib.
One particular type of air bag module is known in the art as a multi-stage system and includes a plurality of actuatable stages. Such a multi-stage air bag module typically includes two or more separate sources of inflation fluid controlled by actuation of associated squibs.
Turning to the condition that results in air bag module actuation, the typical condition is a vehicle crash condition. It is common to use a crash sensor that is an accelerometer to sense the vehicle crash condition. The accelerometer is typically sensitive along a single axis (e.g., a fore-two-aft vehicle axis). Such an accelerometer can have a full-scale sensitivity of 50 G, but may not be sensitive enough to detect pre-crash braking. Often, it may be desirable to detect pre-crash braking (i.e., as a pre-crash cursor to a potential vehicle crash condition). Also, rough roads may effect the accelerometer. The effect imposed upon the accelerometer by rough roads is inversely related to the sensitivity of the accelerometer.
In view of the characteristics of the accelerometer, there is often a need for a redundant "safing" crash sensor in some occupant protection systems. The accelerometer is a primary sensor in such a system, and both the primary and the safing sensor must respond to a condition (e.g., vehicle deceleration) that is indicative of a vehicle crash condition in order for the protection device (e.g., the air bag module) to be actuated.
Turning again to multi-stage air bag systems, often it is desirable to initiate the stages at different times (e.g., second stage initiation subsequent to first stage initiation). It is known to control initiation of the multiple stages based upon a timer function, the idea being that a time period is tolled for a second stage while first stage initiation occurs. In one example, a timer for the second stage is started at a beginning of a crash event. However, it is often difficult to monitor for the beginning of the crash event to start the timer. False starts (and stops) of the timer could occur due to spurious signals generated as the result of road noise.